The present invention relates to the field of mechanically joining composite panels such as in aerospace vehicles, and more particularly, to the use of composite fasteners in which a portion of the fastener is reformed into an upset head for retaining the mechanical connection.
The use of composite materials in the construction of military and commercial aerospace vehicles is widespread and increasing. The most commonly used composites consist of a polymer matrix reinforced with fibers of such materials as carbon and glass.
In many cases relatively thin composite panels are used. To fasten such panels together, adhesive bonding is used wherever possible. When adhesive bonding alone is judged insufficient, fasteners are used in addition to, or in place of, adhesives.
In the manufacture of conventional aluminum aircraft, solid aluminum rivets have been used successfully in very large quantities to fasten thin panels, starting in about 1935 and continuing to the present time. Aluminum rivets are not always suitable, however, for use in composite sheet material for several reasons. Some composite materials cause accelerated corrosion of aluminum rivets. The coefficients of expansion of aluminum and the composite materials may be too widely different. For these reasons a need exists for a rivet which is itself made from composite material.
Considerable work is now being done to enable rivets made from composite material to be used to fasten composite sheets, with or without prior adhesive bonding. Until recently, these composite solid rivets have been fabricated using the injection molding process to form a rivet having the required manufactured head shape at one end of the usual cylindrical shank portion. The shank portion was made sufficiently long to pass through the workpieces to be fastened and then protrude approximately two shank diameters beyond. Later, this two diameter protruding length is upset to form a "shop formed" head, which in conjunction with the manufactured head, serves to hold the workpieces together.
Examples of suitable materials for making composite solid rivets by injection molding are PEI (polyetherimide), reinforced with short glass fibers, and PEEK (polyetheretherketone), reinforced with short carbon fibers. Both these materials belong to a composite materials category which is commonly known as "thermoplastic." Such materials become soft and formable at temperatures in the 600.degree.-700.degree. range, but when cooled to room temperature, exhibit useful structural strength. These materials, therefore, are ideal for "shop formation."
Rivets made from these short fiber reinforced thermoplastic materials have typical average shear strengths of about 16 KSI when measured using the method of MIL-STD-1312 Test 20, and have typical average tension strength in the range of 14 to 16 KSI. These rivets may be upset with any simple tooling capable of providing a heat input to the protruding rivet tail, followed by pressure to upset the tail when it becomes soft and formable.
This riveting process has gained favor for fastening composite materials because the rivets are easily made on high production injection molding machines they are also inexpensive compared to other suitable fasteners for composites such as titanium threaded shear pins and collars. Also, the drilling of holes in a workpiece and the insertion and upsetting of these rivets can be easily automated. In this way, fastening may be accomplished with a simple inexpensive fastener using an inexpensive and reliable installation method.
In some thick or high bearing strength workpieces, the 16 KSI shear strength and the 12 to 14 KSI tension strength of common short fiber reinforced thermoplastic materials is not adequate. Thus, the need exists for a rivet made from composite material and having higher shear and tension strengths.
Long fiber reinforced, threaded shear-pin type fasteners, having 40 to 60 KSI average ultimate shear strength, have been developed by the Cherry Division of Textron, Inc. However, it is difficult to upset rivets made in this way in a workpiece.
Joining of panels using long fiber reformable composite rivets has been described earlier in U.S. patent application Ser. No. 475,155, now abandoned in favor of continuation application Ser. No. 682,880 filed Apr. 8, 1991, wherein the tail of a solid composite rivet protruding through the panels or workpiece is reformed into a head pressing against the panels. The deforming takes place through heating of the rivet tail and ultimately causing it to deform from pressure applied to the softened composite. Heating in this instance is achieved by the friction of a rotating shaping anvil pressing against the rivet tail.
Because the heat generated by the friction of the rotating anvil begins to dissipate when rotation is terminated, the softened rivet head cools and then hardens.
In the past, this method has been better suited for use with a stationary machine rather than a hand-held tool. The stationary unit allows proper alignment of all parts and prevents abuse to the panels through a controlled process comprising, controlling the speed of the rotating anvil, the pressure on the workpiece, and the rate of displacement of the rivet tail.
There are, however, installation situations where such a stationary anvil can not be employed. Thus, it is desirable to be able to use the rotating tool technique with a hand-held tool.
Hand tools have, of course, been used for many years for deforming fastener components without the use of heat. One well-known method of installing blind rivets is to pull a rivet stem partially through a tubular fastener to enlarge the blind end of the fastener and lock it in place. Typically, the use of a break groove or break notch allows separation of a portion of the stem by pulling the stem until the tensile strength of the reduced diameter is exceeded and it fractures at the groove or notch. Also, hand tools are used for deforming nuts onto lock pins having pull stems and break grooves.